1. Of course you get more risks of being affected by storms, tsunamis, etc.
2. Then, you'd have to deal with people or beach-goers - they're not always nice to be around, aren't they? o_O
3. Last, what I don't like about living near the sea is the salt in the air/sea spray whatever.
It doesn't do any good to the skin and the hair and of course to anything metal in the house.
But, honestly, having a house near the sea is a great (and beautiful) privilege. :)
Here, I think the advantages weigh more